1. Field of the Invention
This invention relates generally to devices for visually displaying data and images and, more particularly, to flat planel liquid crystal display devices.
2. Description of the Related Art
Certain materials, generally referred to as liquid crystals, are electro-optic in that an application of a voltage of appropriate magnitude changes the optical properties of light transmitted through the liquid crystal media. This change in the light transmitted through the liquid crystal media has received increasing use as the basis for optical displays. This electro-optic behavior is particularly well suited for active matrix displays where a high impedance solid state switch (such as a thin film transistor or diode) is used to transfer voltage to the liquid crystal capacitor at each pixel of the matrix display. In addition, the liquid crystal display units have a low power requirement, the power being used only to deform the initial orientation of the liquid crystal molecules. Liquid crystal display units can operate in ambient light when a mirror is placed behind the liquid crystal material and the optical properties of the material are used to control the reflected light. The use of ambient light typically requires that the liquid crystal display unit be oriented carefully with respect to the light source. To reduce the orientation problem and to alleviate the requirement for an ambient light, the liquid crystal display panel can be illuminated from the rear. Finally, the liquid crystal display panel can be fabricated to be flat and relatively narrow. With modern technology, the associated circuits can be made to occupy relatively little space within any desired configuration.
However, to the present time, the liquid crystal displays have suffered from the inability to incorporate a satisfactory gray scale capability in the unit. In the past, attempts to achieve a gray scale capability in liquid crystal displays have used the direct approach of dividing each display point, hereinafter referred to as pixel, into a plurality of subunits or subpixels. The number of subunits which are activated provides the gray scale gradations, the eye typically positions at sufficient distance to integrate any image granularity of the pixel. This approach has proven unsatisfactory for two reasons. For any significant number of pixel subunits, the density of address lines is greatly increased with the resulting difficulties in fabrication. In addition, the addressing becomes more difficult because a plurality of subpixels may need to be activated simultaneously for each pixel. Another technique that has been attempted in order to achieve a gray scale is the result of partially deforming the molecular orientation of the liquid crystal material by applying a voltage to the pixel that is greater than the threshold voltage, but less than the voltage for optical saturation. For a twisted nematic liquid crystal display, such a voltage level produces parial deformation of the liquid crystal molecular alignment. However, the partial deformation results in a gray scale which is a function of the viewing angle about an axis perpendicular to the liquid crystal display and, for this reason, has not proven satisfactory. Referring now to FIG. 1, the angular dependence of the transmission of light through a twisted nematic liquid crystal for several applied voltages is illustrated. The applied voltage is stated in terms of the ratio of the applied voltage V to the threshold voltage V.sub.O for V/V.sub.O =3 (FIG. 1A), 2 (FIG. 1B), 1.5 (FIG. 1C) and 0 (FIG. 1D). The angle of viewing (i.e., departure from the axis of the display unit) is 45.degree. and the angle 0 is the angle around the axis (the angles being illustrated in FIG. 1E). The magnitude of transmission is given by the distance from the center (axis) of the FIG. As will be apparent, any attempt to use the applied voltage in a cell containing a twisted nematic liquid crystal to control the gray scale will be unsatisfactory because of the angular dependence of the optical transmission.
A need has therefore been felt for a technique that would provide a gray scale capability for a liquid crystal display unit without the angular dependence associated with prior art liquid crystal display unit gray scale techniques.